Antilock brake control apparatus

ABSTRACT

In the present invention, the conditions necessary for the change from the reducing-pressure mode to the holding pressure mode are changed based on the length of the period of instability from the beginning of wheel slippage to the stopping of this slippage, and the length of the period of continued severe locking in which the slip ratio of the wheel exceeds a certain fixed value. Therefore, even when there is a disturbance in the form of roughnesses in the road surface or the like, this is determined to be a temporary phenomenon and the switching of the setting from the reducing-pressure mode to the holding pressure mode can take place in a reliable manner, and the usual, stable braking power can be achieved in correspondence with the relationship between the road surface and the tires.

.Iadd.This is a re-issue of application 07/610,519, filed Nov. 8, 1990,now U.S. Pat. No. 5,125,723..Iaddend.

This is a re-issue of application 07/610,519, filed Nov. 8, 1990, nowU.S. Pat. No. 5,125,723..Iaddend.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antilock brake control system forautomobiles, and more particularly to a technique which makes possiblethe control of normally adaptable brake pressure in correspondence tothe coefficient of friction between the tires and the road.

2. Prior Art

The antilock brake control apparatus shown in Japanese PatentApplication, first publication, laid open number 60-35647, isconventionally known as an antilock brake control apparatus whichprevents the locking of the wheels of an automobile when braking.

This antilock brake control apparatus has a determining mechanism, whichcompares previously set standard wheel speeds and standard wheelaccelerations with wheel speeds which are detected by means of a wheelspeed sensor or wheel accelerations which are differential valuesthereof, and determines a μ-level (corresponding to the coefficient offriction) between the road surface and the tires, and a changingmechanism, which resets the pressure reduction and stopping level ofbrake fluid pressure when antilock brake controlling is based on theμ-level determined by the determining means.

By means of this, in the case in which the "H" level (the coefficient offriction is large) has been determined by the determining mechanism, thepressure reduction and stopping level is set at a low "H-μ", and thereduction in brake pressure is stopped comparatively early. Furthermore,in the case in which the "L" level (the coefficient of friction issmall) has been determined by the determining mechanism, the pressurereduction and stopping level is set at a high "L-μ", and the reductionin brake pressure is stopped comparatively late.

In an antilock brake control apparatus with the above construction, theμ-level is determined based on the wheel speed, the wheel acceleration,the standard wheel speed and the standard wheel acceleration, or isbased on a slip ratio calculated from the wheel speed and a simulatedbody speed (for example, a body speed inferred from fixed deceleration(tendencies)). In addition, it makes the appropriate setting of thepressure reduction and stopping level of the brake pressure to "H-μ" or"L-μ". However, this type of determining and setting method has aproblem, as shown by a line in FIG. 6, in that when momentary largeslippage occurs as a result of unevennesses in the road surface (forexample, the wheels bounce while the brakes are being applied), althoughthe value should be H-μ, the recovery (reduction) of the slip ratiotakes some time, so that "L-μ" is erroneously determined.

Furthermore, when brake pressure is being decreased, as shown by theline in FIG. 7, when the recovery of the wheel speed is small (theacceleration is small), for example, in the case in which afriction-reducing material such as sand or water is temporarily presenton the road surface, or in the case in which the fluid pressure is beingcontrolled in a manner appropriate for a "H-μ" road surface, or thelike, although the actual value is H-μ, "L-μ" is erroneously determined.

SUMMARY OF THE INVENTION

The present invention was created in view of the above circumstances;when antilock brake control is being carried out, the standards ofdetermination of the change from a state of reducing pressure to a stateof holding pressure are set based on 1) the length of the period ofinstability from the beginning of wheel slippage to the stopping of thisslippage, and 2) the length of the period of continued severe locking inwhich the slip ratio of the wheel exceeds a certain fixed value. Bymeans of this, even when there is a disturbance in the form ofroughnesses in the road surface or the like, this is determined to be atemporary phenomenon and the switching of the setting from thereducing-pressure mode to the holding pressure mode can take place. Itis an object of the present invention to provide an antilock brakecontrol apparatus with the above-mentioned characteristics.

In addition, in order to achieve the above purpose, the inventionincorporates

a vehicle wheel speed sensor;

a modulator to set one mode for the vehicle wheel brake fluid, the modeselected from the group of an increase pressure mode, a decreasepressure mode, and a holding pressure mode,

a controller for controlling said modulator, which makes a decision todetermine if slippage is occurring based on vehicle wheel speed datafrom said vehicle wheel speed sensor, said controller selecting

decreasing pressure mode on the detection of the initiation of vehiclewheel lock,

holding pressure mode on the detection of the termination of vehiclewheel lock, and

increase pressure mode on the detection of vehicle wheel rotation abovea preset level, said controller provided with

a first determining mechanism, said mechanism determining if the periodof instability from the initiation of slippage to the termination ofthat slippage exceeds a previously set first threshold value;

a second determining mechanism, said mechanism determining if the lengthof the period in which severe locking is occurring exceeds a previouslyfixed second threshold value: and

a condition-changing mechanism, to change the conditions necessary forthe transfer from the reducing-pressure mode to the holding pressuremode based on the results of the first and second determiningmechanisms.

According to the invention constructed in this fashion, the conditionsnecessary for the change from the reducing- pressure mode to the holdingpressure mode are changed based on the length of the period ofinstability from the beginning of wheel slippage to the stopping of thisslippage, and the length of the period of continued severe locking inwhich the slip ratio of the wheel exceeds a certain fixed value.Therefore, even when there is a disturbance in the form of roughnessesin the road surface or the like, this is determined to be a temporaryphenomenon and the switching of the setting from the reducing-pressuremode to the holding pressure mode can take place in a reliable manner,and the usual, stable braking power can be achieved in correspondencewith the relationship between the road surface and the tires.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a piping diagram showing the brake fluid system to supplyhydraulic fluid to the wheel cylinder of the brake.

FIG. 2 is a piping diagram showing a provided modulator.

FIG. 3 is a graph which indicates the time variation of the wheel speed,wheel acceleration, and brake fluid pressure, based on the operation ofa valve.

FIG. 4 is a flow chart showing regulation of the brake fluid pressurewhen the wheel acceleration Rω' is decreased or increased beyond a fixedvalue.

FIG. 5 shows a flow chart of the selected decision basis to decide thedecrease pressure stop.

FIG. 6 is a graph showing the relationship of vehicle body speed andwheel speed when a strong slip of the wheel occurs on the road surface.

FIG. 7 is a graph showing the relationship of vehicle body speed andwheel speed in the case in which simultaneous slip is produced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the construction of an antilock brake control apparatuswhich is a preferred embodiment of the present invention will beexplained with reference to FIGS. 1-5.

First, the general construction of the antilock brake control apparatuswill be explained by means of FIGS. 1 and 2.

Numeral 1 indicates the master cylinder; this master cylinder 1 releasesbrake fluid by means of the stepping force on brake pedal 2. The brakefluid released by this master cylinder 1 is supplied through the mediumofmodulators 3 to the wheel cylinders 4 and 5 of brakes (not shown inthe figure) of the left and fight front wheels, and is supplied throughthe medium of fluid pressure control valve 6 to the wheel cylinders (notshownin the figure) of brakes 7 and 8 of the left and right rear wheels.Modulators 3 are provided on the piping systems from master cylinder 1to each wheel cylinder 4, 5, 7, and 8, to regulate rises in brake fluidpressure by means of a control signal supplied by controller 9, and toserve the function of restoring fluid pressure. A detailed descriptionof these modulators 3 is given below.

Furthermore, wheel speed sensors S, which detect peripheral speed, areprovided at each wheel; by means of a control signal supplied bycontroller 9 to modulator 3 based on the wheel speed data obtained bythese wheel speed sensors S, some types of antiskid control areeffected.

Next, the concrete construction of each modulator 3 is explained bymeans of FIG. 2.

Numeral 10 indicates a switching valve, which switches position fromopen to closed; by means of this switching valve 10, the piping systemleading from master cylinder 1 to each wheel cylinder 4, 5, 7, and 8 canbe openedand closed. Furthermore, a check valve 11 is provided at theswitching valve 10; when switching valve 10 is in the "closed" position,the flow offluid in the direction of master cylinder 1 is allowed.

In addition, switching valve 12 is connected in a parallel position withswitching valve 10; this switching valve 12 performs, by means of acontrol signal supplied by controller 9, an opening and closingoperation to release fluid pressure within wheel cylinders 4, 5, 7, and8 to reserve13. Numeral 14 indicates a pump which is driven by motor 15;this pump 14 is driven, by means of a control signal from controller 9,to restore the fluid pressure of the piping system which was lowered atthe time of antiskid control.

By means of the above construction, a modulator 3 is set to one of thefollowing:

a) the increasing-pressure mode, in which switching valve 10 is set to"open" and switching valve 12 is set to "closed",

b) the decreasing-pressure mode, in which switching valve 10 is set to"closed" and switching valve 12 is set to "open", or

c) the holding pressure mode, in which both switching valves 10 and 12are set to "closed";

thus the flow of fluid in the piping between each wheel cylinder 4, 5,7, and 8, and master cylinder 1, is controlled.

Furthermore, in the antilock brake control executed by controller 9,wheel speed Rω and wheel acceleration Rω', which is thedifferentialvalue thereof (however, when decelerating, this valuebecomes negative) arecalculated based on the signals supplied by eachwheel speed sensor S, the slip ratio S is calculated from, for example,simulated wheel speed V, which infers a body speed from a fixeddeceleration value, and wheel speedRω, and the modulators 3 are set tothe increasing-pressure mode, thedecreasing-pressure mode, or theholding pressure mode, based on the results of this calculation.

In other words, basically, in the case in which slip ratio S exceeds apreviously fixed value, or in the case in which wheel acceleration Rω'(wheel speed Rω) decreases beyond a previously fixed value, brake fluidpressure is decreased, and furthermore, in the case in which slip ratioS decreases beyond a previously fixed value and wheel acceleration Rω'(wheel speed Rω) acquires a tendency to increase, brake fluid pressureis increased.

Here, the brake fluid pressure controlling operation in the case inwhich the wiled acceleration Rω' is decreased or increases beyond afixed value will be explained with reference to FIG. 3 and the flowchart of FIG. 4.

Wheel speed Rω and wheel acceleration Rω', which are used as input datain the following explanation, are calculated on the basis of wheel speeddata outputted by wheel speed sensor S, as stated above. Furthermore,the contents of the control shown in the flowchart below are stored inthe above-mentioned controller 9.

Phase 1

SP1: This begins when brake pedal 2 is pressed: it sets theincreasing-pressure mode and increases the brake fluid pressureaccording to the increase in the fluid pressure of master cylinder 1, orby steps.

SP2: A determination is made as to whether a wheel has slipped withrespectto the road surface and wheel acceleration Rω' (wheeldeceleration) has reached a previously fixed reducing-pressure stoppingthreshold value "a", in other words, whether wheel acceleration Rω' hasgone below ×|a|; if the result of this determination is "NO",control isreturned to SP1, and if the result is "YES", control proceeds tothefollowing SP3.

Phase 2

SP3: The increasing-pressure mode set by modulator 3 is reset to thedecreasing-pressure mode, and the brake fluid pressure is decreased. Bymeans of this, the slipping condition of the wheel with respect to theroad surface is rectified and the revolutions of the wheel begin torecover.

SP4: One or the other of the "H-μ" reducing-pressure stopping thresholdvalue (third threshold value) (=a) and the "L-μ" reducing-pressurestopping threshold value (third threshold value) (=b) is selected.

The former "H-μ" reducing-pressure stopping threshold value (=a) isselected when the μ-level (corresponding to the coefficient of friction)between the tie and the road surface is determined to be at the "H"level; on the other hand, the "L-μ" reducing-pressure stopping thresholdvalue (=b) is selected when the μ-level is determined to be at the "L"level (the coefficient of friction is small)(the procedure for selectingone or the other of these reducing-pressure stopping threshold valueswill be explained hereinbelow with reference to FIG. 5).

These reducing-pressure stopping threshold values (a, b) correspond tothe third threshold value shown in the claims.

SP5: A determination is made as to whether the wheel rotation hasrecoveredand wheel acceleration Rω' (wheel deceleration) has reached apreviously fixed reducing-pressure stopping threshold value, in otherwords, whether wheel acceleration Rω' has exceeded -|a| or |b|; if theresult of this determination is "NO", control is returned to SP3, and ifthe result is "YES", control proceeds to the following SP6.

Phase 3

SP6: When the wheel rotation recovers and wheel acceleration Rω' exceedsa previously set reducing-pressure stopping threshold -|a| or |b|, thereducing-pressure mode set by modulator 3 is reset to the holdingpressure mode, and brake fluid pressure is maintained.

SP7: When the holding pressure mode is set by modulator 3, wheelacceleration Rω' gradually stabilizes: however, a determination is madeas to whether at this time, wheel acceleration Rω' goes belowapreviously set threshold value |c|.

By means of this determination, in the case in which Rω' is greater thanthreshold value |c| and the result is "NO", controlreturns to SP6 andthe holding pressure mode continues, in the case in which Rω' is lessthan threshold value c and the result is "YES", control returns to SP1of Phase 1, the holding pressure mode which was setby modulator 3 isreset to the increasing-pressure mode, and the brake fluid pressure isincreased by steps.

Next, the flowchart which selects one or the other of the "H-μ"reducing-pressure stopping threshold value (=a) and the "L-μ"reducing-pressure stopping threshold value (=b), which form the basisfor the determination of the transfer from the reducing-pressure mode tothe holding pressure mode and which are shown in SP4 cf Phase 2, will beexplained with reference to FIG. 5.

In the explanation below, the basis for the determination of the "periodofthe unstable state", from the beginning of wheel slippage to thestopping of this slippage, for the selection of the reducing-pressurestopping threshold value is calculated based on the data which areoutputted by wheel speed sensors S, as described above.

1) The period of the unstable state is the period of Phase 2 and Phase3; the basis for the determination of the selected reducing-pressurestoppingthreshold value is whether or not this period exceeds a fixedvalue (Tuth).

2) The maximum acceleration is shown in the time of the recovery ofwheel speed in Phases 2 and 3; the basis for the determination of theselected reducing-pressure stopping threshold value is whether or notthis maximum acceleration exceeds a fixed value (Rω' maxth).

3) The presence or absence of extremely severe locking is determined, asshown by the prior art in FIG. 6, by the degree of separation of bodyspeed and wheel speed, or in other words, by the size of the slip ratio;the basis for the determination of the selected reducing-pressurestoppingthreshold value is whether or not this slip ratio exceeds afixed value (Sth).

4) The period of the continuation of severe locking is shown by theperiod in which the slip ratio exceeds a fixed value (Sth'; however,Sth'<Sth); the basis for the determination of the selectedreducing-pressure stoppingthreshold value is whether or not this periodexceeds a fixed value (Tdth).

These bases of determination (1)-(4) will be explained hereinbelow bythe flowchart of FIG. 5.

Next, using the bases of determination (1)-(4) and with reference toFIG. 5, the flowchart for the purpose of selecting the reducing-pressurestopping threshold value will be explained.

1. (first determination mechanism)

A determination is made as to whether the period of the unstable state(Tu)is less than the basic unstable state period threshold value (firstthreshold value) (Tuth), in other words whether the period of theunstablestate (Tu) is shorter; if the result of this determination is"YES", control proceeds to step 5, if the result is "NO", controlproceeds to thefollowing step 2.

Step 2. (acceleration determination mechanism)

A determination is made as to whether the maximum wheel acceleration(Rω'max) is greater than the maximum acceleration threshold value(Rω'maxth), which forms the basis, if the result of this determinationis "YES", control proceeds to step 5, if the result if "NO",controlproceeds to the following step 3.

Step 3. (slip ratio determination mechanism)

A determination is made as to whether the slip ratio S is greater thanthe slip ratio threshold value (Sth), which forms the basis; if theresult of this determination is "YES", control proceeds to step 6, ifthe result is "NO", control proceeds to the following step 4. By meansof making a determination as to whether the slip ratio S is greater thanthe slip ratio threshold value (Sth), it can be determined whether awheel has experienced extremely severe locking.

Step 4. (second determination mechanism)

A determination is made as to whether the locking period (Td) exceedsthe severe locking continuing period threshold value (Tdth), which formsthe basis; if the result of this determination is "YES", controlproceeds to step 6, if the result is "NO", control proceeds to thefollowing step 5.

Step 5. (condition-changing mechanism)

The μ-level (corresponding to the coefficient of friction) between theroad surface and the tire is determined to be "H-level", and by thisdetermination, the "H-μ" reducing-pressure stopping function thresholdvalue (=a) is set as the basis for the determination for transferringfromthe reducing-pressure mode to the holding pressure mode.

Step 6. (condition-changing mechanism)

The μ-level (corresponding to the coefficient of friction) between theroad surface and the tire is determined to be "L-level", and by thisdetermination, the "L-μ" reducing-pressure stopping function thresholdvalue (=b) is set as the basis for the determination for transferringfromthe reducing-pressure mode to the holding pressure mode. (The changein brake fluid pressure in the case in which the "L-μ" reducing-pressurestopping function threshold value (=b) is set is shown by the lowestdotted line in FIG. 3.)

In the flowchart in FIG. 5, it is permissible to use, as the calculatedvalues used for the determination of step 1, step 2, step 3, and step 4,the numerical values of the brake fluid cycle at those times, or brakefluid cycle values being used when antilock controlling was previouslycarried out. However, it is preferable that the timing of the flowchartshown in FIGS. 4 and 5 be short and the determination thus conform tothe actual road surface conditions at the time of the determination;therefore, it is preferable to use the numerical values of the brakefluidcycle at the time of the determination.

As explained above, according to the antilock apparatus shown in thepresent embodiment, it is possible to accurately determine the μ-levelbetween the road surface and a tire and set either the "H-μ" reducingpressure stopping threshold value (=a) or the "L-μ" reducing pressurestopping threshold value (=b), by using the following standards:

1) the period of the unstable state,

2) the greatest acceleration,

3) the presence of absence of extremely severe locking, and

4) the period of the continuance of severe locking.

It is thus possible to obtain normal, stable braking force according totherelationship between the road surface and the tires.

By means of this, even in cases in which there are temporaryunevennesses in the road surface causing disturbances, the disturbancesare determined to be temporary, and the setting of a mistaken reducingpressure stopping threshold value is avoided.

In the present preferred embodiment, one of either the "H-μ" reducingpressure stopping threshold value or the "L-μ" reducing pressurestopping threshold value is selected by means of the determinationstandards shown in (1)-(4), but the present invention is not limited tothis; it is permissible to set the reducing pressure stopping thresholdvalue in an analog manner based on the determination standards (1)-(4)(finding the reducing pressure stopping threshold value by calculation,orstoring a number of reducing pressure stopping threshold values inadvance and selecting one).

Furthermore, in the flowchart of the present invention, the reducingpressure stopping threshold value is selected according to the fourdetermination standards shown in (1)-(4), but it is of course possibleto add other determination standards if the speed of the determinationprocessing is not late.

What is claimed is:
 1. An antilock brake control apparatus comprisinga) a vehicle wheel speed sensor; b) a modulator to set one mode for the vehicle wheel brake fluid, the mode selected from the group consisting of an increasing pressure mode, a decreasing pressure mode and a holding pressure mode, c) a controller for controlling said modulator, which makes a decision to determine of slippage is occurring based on vehicle wheel speed data from said vehicle wheel speed sensor, said controller selecting decreasing pressure mode on the detection of the initiation of vehicle slip, holding pressure mode on the detection of the recovery tendency of wheel slip, and increasing pressure mode on the detection of the termination of wheel slip, by comparing said vehicle wheel speed data with a standard threshold value for mode selection, said standard threshold value including a transition threshold value for determining the transition from said .[.holding.]. .Iadd.decreasing .Iaddend.pressure mode to said .[.increasing.]. .Iadd.holding .Iaddend.pressure mode, said transition threshold value being selected from at least two threshold values, High-μ for high frictional level condition and Low-μ for low frictional level condition, said High-μ being set as a lower barrier .[.in case of a.]. .Iadd.to .Iaddend.transition from said .[.holding.]. .Iadd.decreasing .Iaddend.pressure mode to said .[.increasing.]. .Iadd.holding .Iaddend.pressure mode than the Low-μ; .Iadd.said controller having: .Iaddend. means for a first determination, said means comparing the period of instability from the initiation of slippage to the termination of that slippage, with a previously set first threshold value; a means for a second determination, said means comparing the length of the period in which severe wheel slip is occurring with a previously fixed second threshold value; and a means for a third determination, said means comparing a maximum rotational acceleration of the vehicle wheel speed data with a previously fixed acceleration threshold value; and a condition changing means for changing the conditions for the transition from the decreasing pressure mode .[.and.]. .Iadd.to .Iaddend.the holding pressure mode.Iadd., by selecting one out of said at least two threshold values, .Iaddend.based on the results of the means for the first, second and third determinations, wherein said condition changing means selects the High-μ when the first determination means determines that the period of instability from the initiation of slippage to the termination of that slippage is smaller than a previously set first threshold value; or the third determination means determines that the maximum rotational acceleration of the vehicle wheel calculated from the vehicle wheel speed data exceeds the previously fixed acceleration threshold value, and said condition changing means selects the Low-μ when the second determination means determines that the length of the period in which severe wheel slip is occurring exceeds a previously fixed second threshold value.
 2. An antilock brake control apparatus according to claim 1, wherein said standard threshold value is vehicle wheel acceleration.
 3. An antilock brake control apparatus according to claim 1, wherein said Low-μ is greater than or equal to zero, and said High-μ is less than or equal to zero.
 4. An antilock brake control apparatus according to claim 1, wherein said condition changing means selects the Low-μ in the case where a slip ratio exceeds a previously set slip ratio.
 5. An antilock brake control apparatus according to claim 1, wherein said first and third .[.determination is done.]. .Iadd.determinations are performed .Iaddend.based on .[.the.]. data of the previous braking operation. 